Spread-spectrum communication systems are well known in the art and widely deployed. A class of receivers well suited for use in spread-spectrum systems—such as those standardized in IS-95, IS-2000 (cdma2000), and the 3rd-Generation Partnership Project's (3GPP) Wideband Code-Division Multiple Access (W-CDMA) specifications—is the linear interference-whitening (LIW) receiver. LIW receivers suppress interference in addition to collecting signal energy for detection. One form of the LIW receiver is a transversal chip equalizer; another is a G-Rake receiver. The Rake receiver derives its name from its rake-like structure, wherein multiple receiver “fingers” are used to receive multiple signal images in a received multipath signal. By coherently combining the finger outputs in a weighted Rake combiner, the conventional Rake receiver can use multipath reception to improve the Signal to Interference-plus-Noise Ratio (SINR) of the received signal. A Generalized Rake (G-Rake) receiver improves interference suppression performance over a conventional Rake receiver using more sophisticated generation of the combining weights.
Recently, 2×2 Multiple-Input Multiple-Output (MIMO) technology has been standardized in Release 7 of the 3GPP specifications. The standardized scheme, referred to as Dual-Transmit Adaptive Arrays (D-TxAA), is similar to selective per-antenna rate control (S-PARC), except that adaptive unitary precoding is applied to each of the data streams, in this case to each of one or two High-Speed Downlink Shared Channel (HS-DSCH) data streams.
D-TxAA can be viewed as an extension of the previously standardized closed loop mode 1 (CL-1) transmit diversity scheme, in that the precoding vectors (which map a data stream to the multiple transmit antennas) used for each of the D-TxAA data streams are selected from the same codebook used for CL-1. In contrast to CL-1, however, D-TxAA includes two modes of operation—single-stream mode and dual-stream mode. In single-stream mode, one of the four possible precoding vectors from the CL-1 codebook is applied to a single data stream. In dual-stream mode, orthogonal pairs of precoding vectors (again selected from the CL-1 codebook) are applied to the two data streams. The use of precoding has a significant impact on the receiver, and in particular complicates the design of LIW receivers such as Rake receivers.
Earlier versions of the 3GPP W-CDMA specifications (i.e., prior to Release 7) define two transmit diversity modes, CL-1, and an open-loop mode known as STTD. U.S. patent application Ser. No. 10/800,167 (Pub. No. US 2005/0201447), titled “Method and Apparatus for Parameter Estimation in a Generalized Rake Receiver,” filed Mar. 12, 2004 by Cairns et al. (the “Cairns application”), assigned to the assignee of the present application and incorporated herein by reference in its entirety, discloses a solution for G-Rake receivers in a transmit diversity system. The solution describes a parametric approach to estimating an impairment covariance matrix used to form G-Rake combining weights. The parametric approach estimates the impairment covariance matrix as a sum of terms, including a separate term for each transmit antenna as well as a term corresponding to the sum of noise plus other-cell interference.
This solution works well for open-loop transmit diversity modes. In an open-loop mode, the impairments corresponding to each transmit antenna during a particular symbol period are uncorrelated, since different symbols are transmitted from the different antennas. In closed-loop mode, however, the mobile terminal specifies a phase offset, and the same symbol is transmitted by a primary antenna and simultaneously by a secondary antenna with the specified phase offset. In this case, the impairment due to each transmit antenna is highly correlated. This correlation may be exploited to improve interference suppression and receiver performance. U.S. patent application Ser. No. 11/751,109, titled “Receiver Parametric Covariance Estimation for Transmit Diversity,” filed May 21, 2007 by Jonsson et al. (the “Jonsson application”), assigned to the assignee of the present application and incorporated herein by reference in its entirety, discloses a parametric approach to estimating an impairment covariance matrix that accounts for the simultaneous transmission of the same symbols from a first and second antenna. In this approach the impairment covariance matrix for a system employing two transmit antennas is formulated as a sum of seven terms, including a term corresponding to each of the transmit antennas, a noise-plus-other-cell-interference term, plus four additional terms corresponding to the four possible precoding vectors in the CL-1 codebook. The terms are weighted by fitting parameters determined by fitting the parametrically modeled impairment covariance matrix to a measured impairment covariance matrix. An implicit assumption is that if one or more of the preceding vectors are not used by any user in the cell, then the corresponding fitting parameter will ideally be estimated as zero.
The CL-1 covariance estimation approach described in the Jonsson application applies to the transmission of only a single data stream, mapped according to a precoding vector to two (or more) antennas. In contrast, in D-TxAA, two data streams may be transmitted simultaneously, with both data streams sharing the same set of channelization codes. This creates additional self-interference, referred to as code-reuse interference, which affects the formulation of the impairment covariance. Code reuse is not accounted for in the formulation of the Jonsson application, since only one data stream is ever transmitted in CL-1.
In a co-pending patent application titled “Receiver Parametric Covariance Estimation for Precoded MIMO Transmission,” U.S. patent application Ser. No. 12/036,323 (the “Grant application”), the entire contents of which are incorporated by reference herein, a MIMO G-Rake receiver operating at the symbol level is disclosed that is based upon the most general G-Rake formulation for MIMO. For a 2×2 MIMO scenario, this receiver computes an impairment covariance matrix according to:Ru=α1R11+α2R22+α12+R12++jα12−R12−βRn,  (1)where R11 captures the interference due to a first transmit antenna, R22 captures the interference due to a second transmit antenna, R12+ and R12− represent cross-antenna interference, and Rn accounts for white noise passing through the receive filter. The weighting terms are given by:
                                                                        α                1                            =                            ⁢                                                1                                                                                    γ                        p                                            ⁡                                              (                        1                        )                                                              ⁢                                          N                      p                                                                      [                                                                            Γ                                              D                        /                        P                                                              ⁡                                          (                                                                        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 (                              1                              )                                                                                ⁢                                                                                                                                                  b                                21                                                                                                                    2                                                                          +                                                                                                            γ                              s                                                        ⁡                                                          (                              2                              )                                                                                ⁢                                                                                  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                 ⁢                                                                                            γ                          v                                                ⁡                                                  (                          ⅈ                          )                                                                    ⁢                                                                                                                              v                                                          2                              ⁢                              i                                                                                                                                2                                                                                            +                                                                            γ                      o                                        ⁡                                          (                      2                      )                                                        ⁢                                      Γ                                          O                      /                      P                                                                      +                                                      γ                    p                                    ⁡                                      (                    2                    )                                                              ]                                                                                          α                12                +                            =                            ⁢                                                1                                                            N                      p                                        ⁢                                                                                                                        γ                            p                                                    ⁡                                                      (                            1                            )                                                                          ⁢                                                                              γ                            p                                                    ⁡                                                      (                            2                            )                                                                                                                                              [                                                      Γ                                          D                      /                      P                                                        (                                                                                                              γ                          s                                                ⁡                                                  (                          1                          )                                                                    ⁢                                              Re                        [                                                                              b                            11                                                    ⁢                                                      b                            21                            *                                                                          ]                                                              +                                                                                                                                                                              ⁢                                                                                    γ                        s                                            ⁡                                              (                        2                        )                                                              ⁢                                          Re                      [                                                                        b                          12                                                ⁢                                                  b                          22                          *                                                                    ]                                                        )                                +                                                      Γ                                          V                      /                      P                                                        ⁢                                                            ∑                                              i                        =                        1                                                                    K                        v                                                              ⁢                                                                                            γ                          v                                                ⁡                                                  (                          ⅈ                          )                                                                    ⁢                                              Re                        ⁡                                                  [                                                                                    v                                                              1                                ⁢                                i                                                                                      ⁢                                                          v                                                              2                                ⁢                                i                                                            *                                                                                ]                                                                                                                                ]                                                                                          α                12                -                            =                            ⁢                                                1                                                            N                      p                                        ⁢                                                                                                                        γ                            p                                                    ⁡                                                      (                            1                            )                                                                          ⁢                                                                              γ                            p                                                    ⁡                                                      (                            2                            )                                                                                                                                              [                                                      Γ                                          D                      /                      P                                                        (                                                                                                              γ                          s                                                ⁡                                                  (                          1                          )                                                                    ⁢                                              Im                        [                                                                              b                            11                                                    ⁢                                                      b                            21                            *                                                                          ]                                                              +                                                                                                                                                                              ⁢                                                                                    γ                        s                                            ⁡                                              (                        2                        )                                                              ⁢                                          Im                      [                                                                        b                          12                                                ⁢                                                  b                          22                          *                                                                    ]                                                        )                                +                                                      Γ                                          V                      /                      P                                                        ⁢                                                            ∑                                              i                        =                        1                                                                    K                        v                                                              ⁢                                                                                            γ                          v                                                ⁡                                                  (                          ⅈ                          )                                                                    ⁢                                              Im                        ⁡                                                  [                                                                                    v                                                              1                                ⁢                                i                                                                                      ⁢                                                          v                                                              2                                ⁢                                i                                                            *                                                                                ]                                                                                                                                ]                                                                          β              =                            ⁢                              N                0                                                                        (        2        )            
Here, Np is the pilot code spreading factor, γx (k) is the fraction of the total base station chip energy allocated to voice (x=v), data (x=s), overhead (x=o), or pilots (x=p) for antenna/stream k, Γx/P is the ratio of chip energies Ex/Ep, bij is the i,j-th element of pre-coding matrix B, and νij is the i-th element of the pre-coding vector v for the j-th voice user. Note that in WCDMA, the columns of B and the pre-coding vectors v are drawn from the codebook Φ={φ1,φ2,φ3,φ4}, where φk=[1 ej(2k-1)π/4 ]T for k=1, 2, 3, 4. The columns of B are chosen from orthogonal pairs of Φ in dual stream mode, whereas only one pre-coding vector is chosen for the first column of B in single stream mode, while the second column is set to the zero vector ([0 0]T).
The G-Rake receiver described in the Grant application utilizes the impairment covariance matrix and net channel estimates to compute combining weights. The combining weights for this receiver structure depend on whether one or two streams are being transmitted. For single stream mode, the combining weights wsingle are obtained by solving the following system of equations:Ruwsingle=h(b),  (3)where the notation h(b) indicates the “effective” net channel coefficients that depend on the pre-coding vector b. (b is the first column of B as described above for single stream mode.)
For dual stream mode, two sets of combining weights (w1dual,w2dual) must be computed. These weights may be obtained by solving the following systems of equations
                                                                                                              (                                                                  R                        u                                            +                                                                                                    α                            PC                                                    ⁡                                                      (                            1                            )                                                                          ⁢                                                  h                          ⁡                                                      (                                                          b                              2                                                        )                                                                          ⁢                                                                              h                            H                                                    ⁡                                                      (                                                          b                              2                                                        )                                                                                                                )                                    ⁢                                      w                    1                    dual                                                  =                                  h                  ⁡                                      (                                          b                      1                                        )                                                                                                                                                                (                                                                  R                        u                                            +                                                                                                    α                            PC                                                    ⁡                                                      (                            2                            )                                                                          ⁢                                                  h                          ⁡                                                      (                                                          b                              1                                                        )                                                                          ⁢                                                                              h                            H                                                    ⁡                                                      (                                                          b                              1                                                        )                                                                                                                )                                    ⁢                                      w                    2                    dual                                                  =                                  h                  ⁡                                      (                                          b                      2                                        )                                                                                      .                            (        4        )            Here, h(bn) is the effective net channel coefficient vector due to pre-coding for stream n, and αPC (n) is a per-code scaling factor that multiplies the outer product of the effective net coefficients to account for the code reuse interference (note: pre-coding vector b1 corresponds to the first column of matrix B while pre-coding vector b2 corresponds to the second column).
Symbol estimates for either single or dual stream mode are obtained by computing the inner product of the combining weights for the given stream with the despread traffic symbols.
As noted above, the MIMO G-Rake receiver formulation disclosed in the Grant application is the most general formulation. However, this solution is quite complex. Four fundamental matrix terms must be computed: R11, R22, R12, and Rn. (The matrix term R12 is used to compute R12+ and R12− for Equation (1)). Of these fundamental terms, R11, R22, and Rn are conjugate symmetric, so only slightly more than half of the matrix elements must be calculated. R12, on the other hand, is not conjugate symmetric, so all matrix elements must be computed. Those skilled in the art will appreciate that the calculations required to compute these matrix terms represent a considerable computation burden.
In addition, the formulation of Equation (1) requires that five parameters be estimated to form the impairment covariance matrix. This is compared to the estimation of only two parameters in a “baseline” non-MIMO G-Rake receiver. Even in these simpler receivers, post-estimation smoothing or other adjustment (e.g. clipping) of parameters has been found useful to obtain good overall receiver performance. Joint estimation of five parameters is likely to require similar (and likely even more complex) post-processing to yield good receiver performance.